If the target land is an illegal target when Sundering Eruption tries to resolve, it won't resolve and none of its effects will happen. That land's controller won't get to search for a basic land card, and creatures without flying will still be able to block this turn.

If the target land isn't destroyed by Sundering Eruption's effect (perhaps because the land has indestructible), its controller still gets to search for a basic land card, and creatures without flying still can't block this turn.

The effect of Sundering Eruption prevents all creatures without flying from blocking that turn, including creatures that lose flying after Sundering Eruption resolves and creatures without flying that enter the battlefield later in the turn. Creatures without flying that gain flying later in the turn will be able to block if they have flying when blockers are being declared.

To determine whether it is legal to play a modal double-faced card, consider only the characteristics of the face you're playing and ignore the other face's characteristics. For example, if an effect stops you from casting creature spells, you can't cast Disciple of Freyalise, but you can still play Garden of Freyalise.

If an effect allows you to play a specific modal double-faced card, you may cast it as a spell or play it as a land, as determined by which face you choose to play. If an effect allows you to cast (rather than "play") a specific modal double-faced card, you can't play it as a land.

If an effect allows you to play a land or cast a spell from among a group of cards, you may play or cast a modal double-faced card with any face that fits the criteria of that effect. For example, if an effect allows you to play lands from your graveyard, you can play Garden of Freyalise, but you can't cast Disciple of Freyalise.

If an effect allows you to put a card with particular characteristics onto the battlefield without instructing you to play or cast it, you consider only the characteristics of a modal double-faced card's front face to see if that card qualifies. If it does, it enters the battlefield with its front face up. For example, if an effect allows you to put a creature card from your graveyard onto the battlefield, you can put Disciple of Freyalise onto the battlefield. However, an effect that lets you return a land card from your graveyard to your hand won't let you return Garden of Freyalise to your hand, as that card has only its front face's characteristics while in the graveyard.

The mana value of a modal double-faced card is based on the characteristics of the face that's being considered. On the stack or the battlefield, consider whichever face is up. In all other zones, consider only the front face. This is different than how the mana value of a transforming double-faced card is determined.

A modal double-faced card can't be transformed or be put onto the battlefield transformed. Ignore any instruction to transform a modal double-faced card or to put one onto the battlefield transformed.